The invention relates to a steam generator in which a once-through heating area is arranged in a heating-gas duct through which flow can occur in an approximately horizontal heating-gas direction, which once-through heating area comprises a plurality of steam generator tubes connected in parallel to the throughflow of a flow medium and which is designed in such a way that a steam generator tube heated to a greater extent compared with a further steam generator tube of the same once-through heating area has a higher rate of flow of the flow medium compared with the further steam generator tube.
In a gas- and steam-turbine plant, the heat contained in the expanded working medium or heating gas from the gas turbine is used for generating steam for the steam turbine. The heat transfer is affected in a heat-recovery steam generator which is connected downstream of the gas turbine and in which a plurality of heating areas for water preheating, for steam generation and for steam superheating are normally arranged. The heating areas are connected in the water/steam circuit of the steam turbine. The water/steam circuit normally comprises several, e.g. three, pressure stages, in which case each pressure stage may have an evaporator heating area.
For the steam generator connected as a heat-recovery steam generator downstream of the gas turbine on the heating-gas side, a plurality of alternative design concepts come into consideration, namely the design as a once-through steam generator or the design as a circulation steam generator. In a once-through steam generator, the heating of steam generator tubes provided as evaporator tubes leads to an evaporation of the flow medium in the steam generator tubes in a single pass. In contrast thereto, in a natural- or forced-circulation steam generator, the circulating water is only partly evaporated when passing through the evaporator tubes. The water which is not evaporated in the process is fed again to the same evaporator tubes for a further evaporation after separation of the generated steam.
A once-through steam generator, in contrast to a natural- or forced-circulation steam generator, is not subject to any pressure limit, so that live-steam pressures are possible well above the critical pressure of water (Pcri≅221 bar)— where there is only a slight difference in density between a medium similar to a liquid and a medium similar to steam. A high live-steam pressure promotes a high thermal efficiency and thus low CO2 emissions of a fossil-fired power plant. In addition, a once-through steam generator has a simple type of construction compared with a circulation steam generator and can therefore be manufactured at an especially low cost. The use of a steam generator designed according to the once-through principle as a heat-recovery steam generator of a gas- and steam-turbine plant is therefore especially favorable for achieving a high overall efficiency of the gas- and steam-turbine plant in a simple type of construction.
Particular advantages with regard to the cost of manufacture, but also with regard to necessary maintenance work, are offered by a heat-recovery steam generator in a horizontal type of construction, in which the heating medium or heating gas, that is to say the exhaust gas from the gas turbine, is conducted through the steam generator in an approximately horizontal direction of flow. In a once-through steam generator in a horizontal type of construction, however, the steam generator tubes of a heating area, depending on their positioning, may be subjected to heating that differs greatly. In particular in the case of steam generator tubes connected on the outlet side to a common collector, different heating of individual steam generator tubes may lead to a union of steam flows having steam parameters differing greatly from one another and thus to undesirable efficiency losses, in particular to comparatively reduced effectiveness of the relevant heating area and consequently reduced steam generation. In addition, different heating of adjacent steam generator tubes, in particular in the region where they open into collectors, may result in damage to the steam generator tubes or the collector. The use, desirable per se, of a once-through steam generator of horizontal type of construction as a heat-recovery steam generator for a gas turbine may therefore entail considerable problems with regard to a sufficiently stabilized flow guidance.
EP 0944 801 B1 discloses a steam generator which is suitable for being designed in a horizontal type of construction and in addition has the aforesaid advantages of a once-through steam generator. To this end, the known steam generator is designed with regard to its once-through heating area in such a way that a steam generator tube heated to a greater extent compared with a further steam generator tube of the same once-through heating area has a higher rate of flow of the flow medium compared with the further steam generator tube. The once-through heating area of the known steam generator therefore exhibits a self-stabilizing behavior like the flow characteristic of a natural-circulation evaporator heating area (natural-circulation characteristic) when individual steam generator tubes are heated to a different extent, and this behavior, without the need for exerting an external influence, leads to adaptation of the outlet-side temperatures even on steam generator tubes heated to a different extent and connected in parallel on the flow-medium side. However, the known steam generator is comparatively complicated from a design point of view, in particular with regard to the water- and/or steam-side distribution of the flow medium. In addition, problematic differential expansions may occur between adjacent evaporator tubes and may lead to inadmissible thermal stresses and thus to damage to tubes and collectors.